The present invention is directed to a dual phase structured (ferrite/martensite) steel sheet product and a method of producing the same. In particular, the steel sheet has an excellent combination of high tensile strength and formability, as determined by the strain hardening exponent, namely the n-value.
The following abbreviations are employed here.
ABBREVIATIONSCentigradeC.compact strip productionCSPdegree°FahrenheitF.mega PascalMPamillimetermmpercent%secondsweightwt
Applications of high strength steel sheets to automotive parts, electric apparatus, building components and machineries are currently increasing. Among these high strength steels, dual phase steel, which possess microstructures of martensite islands embedded in a ferrite matrix, is attracting more and more attention due to such dual phase steel having a superior combination of the properties of high strength, excellent formability, continuous yielding, low yield ratio and/or high work hardening. Particularly with respect to automotive parts, martensite/ferrite dual phase steels, because of these properties, can improve vehicle crashworthiness and durability, and also can be made thin to help to reduce vehicle weight as well. Therefore, martensite/ferrite dual phase steels help to improve vehicle fuel efficiency and vehicle safety.
The previous research and developments in the field of dual phase steel sheets have resulted in several methods for producing dual phase steel sheets, many of which are discussed below.
U.S. Patent Application Publication No. 2003/0084966 A1 to Ikeda et al. discloses a dual phase steel sheet having low yield ratio, and excellence in the balance for strength-elongation and bake hardening properties. The steel contains 0.01-0.20 mass % carbon, 0.5 or less mass % silicon, 0.5-3.0 mass % manganese, 0.06 or less mass % aluminum, 0.15 or less mass % phosphorus, and 0.02 or less mass % sulfur. The method of producing this steel sheet includes hot rolling and continuous annealing or galvanization steps. The hot rolling step includes a step of completing finish rolling at a temperature of (Aγ3-50)° C. [sic, (Ar3-50)° C.] or higher; and a step of cooling at an average cooling rate of 20° C./s or more down to the Ms point (defined by Ikeda et al. as the matrix phase of tempered martensite) or lower, or to the Ms point or higher and the Bs point (defined by Ikeda et al. as the matrix phase of tempered bainite) or lower, followed by coiling. The continuous annealing step includes a step of heating to a temperature of the A1 point or higher and the A3 point or lower; and a step of cooling at an average cooling rate of 3° C./s or more down to the Ms point or lower; and, optionally, a step of further applying averaging at a temperature from 100 to 600° C.
U.S. Pat. No. 6,440,584 to Nagataki et al. is directed to a hot dip galvanized steel sheet, which is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700° C. or less, and hot dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3. A continuous hot dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850° C., cooling the soaked strip to a temperature range of 600° C. or less at a cooling rate of 1 to 50° C./s, hot dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600° C. is within 200 s.
U.S. Pat. No. 6,423,426 to Kobayashi et al. relates to a high tensile hot dip zinc coated steel plate having a composition comprising 0.05-0.20 mass % carbon, 0.3-1.8 mass % silicon, 1.0-3.0 mass % manganese, and iron as the balance. The steel is subjected to a primary step of primary heat treatment and subsequent rapid cooling to the Ms point or lower, a secondary step of secondary heat treatment and subsequent rapid cooling, and a tertiary step of galvanizing treatment and rapid cooling, so as to obtain 20% or more by volume of tempered martensite in the steel structure.
U.S. Pat. No. 4,708,748 (Divisional) and U.S. Pat. No. 4,615,749 (Parent), both to Satoh et al., disclose a cold rolled dual phase structure steel sheet, which consists of 0.001-0.008 weight % carbon, not more than 1.0 weight % silicon, 0.05-1.8 weight % manganese, not more than 0.15 weight % phosphorus, 0.01-0.10 weight % aluminum, 0.002-0.050 weight % niobium and 0.0005-0.0050 weight % boron. The steel sheet is manufactured by hot and cold rolling a steel slab with the above chemical composition and continuously annealing the resulting steel sheet in such a manner that the steel sheet is heated and soaked at a temperature from α→γ transformation point to 1000° C. and then cooled at an average rate of not less than 0.5° C./s but less than 20° C./s in a temperature range of from the soaking temperature to 750° C., and subsequently at an average cooling rate of not less than 20° C./s in a temperature range of from 750° C. to not more than 300° C.
The disclosures of all patents and published patent applications, which are mentioned here, are incorporated by reference.
All of the above patents and the above patent publication are related to the manufacture of dual phase steel sheets using a continuous annealing method. Compared to batch annealing, continuous annealing can provide steel sheets which exhibit more uniform mechanical properties. However, the formability and drawability of continuous annealed steel sheets are generally inferior to the formability and drawability of steel sheets produced by batch annealing. A need is thus still called for to develop a new manufacturing method to produce dual phase steel sheets. This appears particularly necessary in North America, where a number of steel manufacturers have no continuous annealing production lines to perform controlled cooling.
The present invention thus has, as a principal object, the provision of a batch annealing method, which typically has less demanding processing requirements than continuous annealing methods, and which advantageously provides a steel sheet that exhibits improvements over the above-described problems of the prior dual phase steel sheet as well as such prior steel sheet having a coating of zinc or a coating of zinc alloy. The batch annealing method should be able to be carried out by most steel manufacturers, using a facility less restrictive than the currently used continuous annealing facilities.